1. Field of the Invention
The present invention is directed towards an exposure projection apparatus, which is employed in lithographic processes to manufacture microdevices such as semiconductor elements, liquid crystal display elements, thin-film magnetic heads, and imaging elements (CCD's), etc., and specifically is directed towards an optical projection apparatus for projecting device patterns onto a photosensitive substrate.
2. Related Background Art
Exposure projection apparatuses used in lithographic processes for manufacturing semiconductor devices are equipped with optical projection systems in which numerous lens elements are mounted inside a barrel. FIG. 7 shows the structure of an optical projection system used in a conventional exposure projection apparatus.
In the optical projection system 80 shown in FIG. 7, a plurality of lens elements 82 are accommodated inside a barrel 81. The plurality of lens elements 82 are held by respective lens cells 84 inside the single barrel 81, and are arranged along an optical axis AX. In order to allow adjustment of the respective positions of the plurality of lens elements inside the barrel 81, e.g., the positions of the first through fourth lens elements as seen from the upper end (reticle R end) of the barrel, along the direction of the optical axis AX, the lens cells 84 that holds these four lens elements are supported by respective spacing adjustment washers 86. A retainer 85 is screwed into the top part of the barrel 81, so that retainer 85 presses against the lens frame 84 located closest to the reticle R. As a result, the plurality of lens cells 84 are respectively fastened in place inside the barrel 81.
However, in an exposure projection apparatus, it may be necessary to make corrections for various aberrations in the optical projection system in response to changes in the working environment. In order to make such corrections it is necessary to alter the spacing of at least two of the plurality of lens elements 82. In the case of the optical projection system shown in FIG. 7, however, it is impossible, due to structural limitations, to alter the spacing of at least two of the plurality of lens elements 82 from outside of the barrel 81 after the optical projection system has been assembled. Accordingly, in order to alter the spacing of at least two of a plurality of lens elements 82, it is necessary to disassemble the optical projection system 80. This involves removing the optical projection system 80 from the exposure projection apparatus, removing the retainer 85 from the barrel 81, removing at least one lens frame 84 from the barrel, and replacing or removing one or more space adjustment washers 86. Accordingly, the time required to disassemble and assemble the optical projection system in order to make adjustments for aberrations becomes problematic. An additional problem also arises in that after the optical projection system, which has been adjusted for an aberration, has been reassembled inside the exposure projection apparatus the positional adjustment and alignment of the optical projection system must be performed once again.
In U.S. Pat. No. 5,117,255, hereby incorporated by reference, a technique is disclosed in which the spacing of several lens elements in a optical projection system is altered in order to correct for superimposition error caused by image distortion in cases where different patterns are superimposed on a wafer using a plurality of exposure projection apparatuses. The spacing of the lens elements is adjusted by supporting a plurality of lenses close to the reticle by means of respective piezoelectric-elements and driving the piezoelectric-elements along the optical axis of the optical projection system.
In recent years, as a result of the increased precision requirements of circuit patterns in semiconductor elements, it has become necessary to achieve ideal imaging by making high-precision adjustments for various types of aberrations in optical projection systems. Furthermore, various aberrations in optical projection systems change according to changes in the reticle pattern and exposure conditions (e.g., size and shape of the secondary light source). Accordingly, there is a need for a mechanism which makes it possible to adjust the spacing of the lens elements in a optical projection system in accordance with the exposure conditions without disassembly of the optical projection system.
Accordingly, in U.S. patent application Ser. No. 08/302,477, filed Sep. 12, 1994, now abandonded, the present Applicants disclosed an exposure projection apparatus in which a) at least one optical element is accommodated in each of a plurality of barrel units; and b) the plurality of barrel units are designed so that these units can be freely attached and detached from the optical projection system, thus allowing easy adjustment of the optical performance values (e.g., magnification, distortion, etc.) of the optical projection system.